1. Field of the Invention
The present invention relates to an electronic logic circuit. More particularly, the present invention relates to an improved logic voting circuit.
A voting circuit is a logic circuit of the type that provides an output which follows the majority of logic levels, such as two-out-of-three input logic levels. A logic equation for such a device is as follows: EQU F=AB+AC+BC
where F is the output of the function described and A, B, and C are the logical voting input signals.
The importance of logic voting circuits is well known. Such circuits accomplish important Boolean functions and therefore find ready application in the computer and electronic control arts. For example, voting circuits provide a means for implementing redundancy in high security devices where results along a single pathway are inadequate or insufficient. A high reliability logic function wherein polling of results, reducing the possibility of spurious responses or indications, thus obtains, thereby providing a failsafe redundant signal pathway. Additionally, integrated circuit yields are substantially improved by providing redundant circuit pathways wherein defective pathways are readily eliminated, thereby repairing an otherwise useless integrated circuit without impairing circuit performance.
2. Description of the Prior Art
One known technique for accomplishing voting uses AND and OR digital logic function circuits coupled in a voting circuit. A prior art voting circuit is shown in FIG. 1. Three miscellaneous logical circuits 11, 12, and 13 with functions designated X, Y, and Z, respectively, serve as inputs. These logical circuits each include corresponding control terminal inputs having values designated T, U, and V. Three corresponding logical output signals A, B, and C are generated on corresponding output lines.
The logical outputs A, B, and C are coupled to three AND gates 14, 15, and 16, the outputs of which are coupled to an OR gate 17 which produces a voted signal output F. The inputs to the AND gates are arranged such that an agreement between the levels of any two logical outputs (A, B, or C) produces a signal which, in turn, via OR gate 17, produces the voted output signal F.
Such known voting circuits require a plurality of digital AND gates, coupled to an OR gate, and have an overhead in function count (i.e. gates per function) that has heretofore not been easily reducible. Because two levels of logic are required for voting--the AND function and the OR function--there is also a signal propagation delay that affects circuit response time. The function count (number of gates) imposed by a two level voting circuit also imposes a power penalty for devices incorporating such voting circuits.
Known voting circuits also have relatively poor noise immunity. A spurious voting result is often produced by power supply voltage drops, power supply noise, and bias voltage noise. Additionally, a partial component failure (e.g., a partial transistor failure in a discrete circuit or monolithic device) can produce a continually wrong result by operating at other than narrowly defined critical digital logic levels. Thus, known voting circuits are level intolerant. In monolithic devices function yield is seriously degraded by such level intolerance where ambiguous results require that the device be discarded. Such devices, once fabricated, cannot be repaired.
Expanding such circuits to vote other odd numbers, such as three-out-of-five, exacerbates the function count overhead, requires additional circuit board space, imposes an additional power penalty, and makes the generation of incorrect results due to noise and other transient signal interference more likely.
Another shortcoming of known voting circuits is limited logic adaptability. If the circuit is operated at a predetermined logic standard, such as a high-equals-true circuit, it cannot readily be changed to a complement function. Such lack of versatility unnecessarily complicates the design task of the engineer. A voting circuit, which minimizes function count (number of gates) and power consumption while providing adequate noise immunity for its application, would be an advance in the art. A circuit additionally incorporating a complement option and voting expansion, while providing adequate noise immunity and repair capability, would be a significant advance in the art.